1. Field of the Invention
This invention relates to the field of conveyor systems and, in particular, to an improved chain guide that offers improved durability and uniformity of positioning of chain guide strips.
2. Description of the Related Art
Conveyor systems are commonly employed in manufacturing applications to transport objects between different processing stations and locations. A conveyor system typically includes a conveyor chain that is arranged in an endless loop and driven about a vertically arranged plane to transport the objects on the chain surface along a generally horizontal, piecewise linear path. Guide rail assemblies are typically placed adjacent and generally parallel with the conveyor chain to retain the objects on the chain surface.
Metal conveyor chains were developed more than 30 years ago and plastic versions followed shortly thereafter. Typical chains include paired tabs or bevels extending inward or outward on each chain segment to retain the chain segments as they travel along a curve of the conveyor track. Each chain segment is typically connected to adjacent segments via integrally formed barrel sections similar to a hinge. An interconnecting pin passes through the barrel sections of adjacent segments thereby joining the segments and forming the endless chain. This hinged chain segment design allows the chain segment top surfaces to remain in a flat plane as the chain is drawn through appropriately spaced guides. The chain guides laterally and vertically locate the chain segments and thus the chain.
The relative placement of various processing stations in a manufacturing application often require that the conveyor system have curves. To provide the most trouble-free operation, the preferred curve is a circle segment. The circle segments preferably intersect either other circle segments or straight runs such that the tangent of the terminal ends of the circle segments are parallel with and intersect adjoining straight segments or other circle segment tangents.
As previously mentioned, the chain segments include either outwardly or inwardly extending tabs for locating the chain segments on a chain guide in a male/female or female/male arrangement respectively. Inwardly extending tabs are generally positioned on the outside edge of the chain segment and thus the chain segments are guided by the edges and supported in the middle.
As the chain typically moves at a high rate of speed and often supports a relatively heavy object load, the material for the chain guide is preferably of high strength and high inherent lubricity. Certain plastics offer a combination of high strength and high inherent lubricity as well as being readily able to be formed into relatively complex shapes. A widely practiced type of chain guide includes individual shaped plastic chain guides that have a generally “L” shaped or rectangular crosssection. These guides can be readily preformed into highly regular circle segments as well as straight segments. However, several difficulties arise in the practice of this chain guide style.
The individual chain guides are typically attached to an underlying metal framework both to physically support the chain guides and to position matched pairs in an equally spaced/parallel arrangement. It will be understood that variations in uniform spacing between opposed guides or unevenness in the curvature can lead to tight or loose sections in the chain guide. Tight sections cause excessive wear at that location and can even lead to the chain binding. Loose sections also lead to excess wear as well as increased noise and vibration. In extreme cases a loose chain guide section can allow the chain to become derailed or vibrate enough to dislodge objects carried on the conveyor.
The chain guides are typically attached either from the sides or above with common fasteners to the metal support framework. The primary difficulties in achieving uniformly spaced, smoothly curved circle segments arise from the construction of the metal framework and the attachment of the guides thereto. Firstly, it is very difficult in practice to accurately form accurate circle segments of the size required for typical conveyor systems. The radius of curvature is typically on the order of feet. Metal circle segments, either of sheet metal, bars, or angles, can become distorted from a perfect circle during handling and assembly.
A second difficulty in configurations where the guides are attached from the side is that the fasteners used to attach the guides to the framework typically have a planar attachment surface whereas the plastic guide has a curved surface. As the fastener is tightened, the curved plastic is forced to flatten in the region of attachment. This leads to a lack of smooth curvature along the curved section of guide.
A third difficulty arises from the typical attachment of plastic to metal with threaded fasteners. While plastic has advantageous properties of strength, lubricity, and ability to be readily formed into curves, it is not well suited to be secured with metal fasteners. It should be noted that conveyor systems are typically run almost continuously at high speeds and with significant weight and vibration loads. This physical stress tends to cause the plastic to cold swage and loosen from the fasteners. It will also be appreciated that only a very limited amount of tightening torque can be applied to plastic as compared to other materials.
As an alternative, a one piece plastic chain guide has been developed that overcomes some of the difficulties previously mentioned. The one piece chain guide defines a semi-enclosed rectangular box cross-section and chain segments with outwardly extending tabs run inside the guide. The one piece chain guide is preformed in circle segments and straight segments thereby eliminating the spacing and uniform curvature problems of the separate guides as previously described.
However, there remain several drawbacks to the one-piece chain guides. Firstly, the one-piece guides are relatively expensive to manufacture. In addition, as a one-piece unit, they have to be replaced in complete segments rather than as individual sides as is possible with separate, opposed chain guide strips. Opposing sides of the strips tend to wear unevenly, particularly in curved sections and replacing entire sections rather than a single side results in increased maintenance costs for conveyor systems based on one-piece chain guides.
A further drawback to the one-piece chain guides is that with a chain installed thereon, an essentially closed space is formed. This closed space readily accumulates debris and makes cleaning the conveyor system extremely difficult. It will be appreciated that for a conveyor system used in a food or medical supply process cleanliness is very important. The closed chain guide also inhibits access to the chain itself greatly reducing the convenience and serviceability of such a system. Finally, the previously mentioned difficulties with attaching a plastic assembly to a metal framework still apply to the one-piece chain guide.
From the foregoing, it will be appreciated that there is a need for a chain guide for conveyor systems that can reliably and inexpensively maintain two opposing chain guides in equally spaced arrangement and maintain a smooth circle segment curvature. There is also a need for a chain guide that avoids enclosure of the chain to facilitate cleaning and access to the chain segments. There is also a need for a chain guide that avoids the need to secure threaded fasteners into plastic material.